1. Field of the Invention
This invention relates to a method for the production of a cast of titanium or a titanium-based titanium alloy having no internal faults by mainly using sponge titanium, mill titanium bars, or titanium scraps as a main raw material, fusing the main raw material, and pouring the resultant melt in a mold.
2. Description of the Prior Art
As widely known, titanium or a titanium-based alloy is exceedingly active in a molten state and unusually susceptible to oxidation during the process of fusing or casting. For the production of a cast of titanium or a titanium-based alloy, therefore, the skull fusing method using an electron beam furnace, a vacuum arc furnace, or a vacuum induction furnace is employed.
In the fusion of the raw material in accordance with the skull fusing method, however, there arises the disadvantage that the melt in the furnace is not fully degasified and the produced cast tends to contain numerous pore defects referred to as gas porosity or microporosity notwithstanding the melt is held under such a high degree of vacuum as 10.sup.-6 Torr as in the electron beam furnace or under a reduced pressure of argon gas from 400 to 600 Torr as in the vacuum arc furnace or the vacuum induction furnace.
The disadvantage of this, nature may be alleviated, for example, by improving the schedule of casting as by increasing the feeder head or enlarging the gate. The melt entraining pore faults may be mended by being squeezed with hot isostatic pressure (HIP) and consequently deprived of the pore faults. Further, by adopting the method of centrifugal casting which brings about the effect of a feeder head by virtue of centrifugal force, there may be obtained a cast possessing a dense texture containing only very few pore faults.
The increase of the feeder head or the enlargement of the gate, however, involves waste of the melt and inevitably impairs the yield of casting and, at the same time, compels the subsequent treatment to demand time and labor. Further, the hot isostatic press (HIP) entails the disadvantage that the process of treatment required for its operation is complicated to a great extent. The centrifugal casting necessitates special devices as for rotation and retention of the mold and incurs the problem of a high cost of equipment.